Analysis | Why the Vaccinated Are Still at Risk From Coronavirus

In clinical trials for the Covid vaccines now in use, efficacy rates ranged from 50% to 95%. Those rates mean that in the group of trial volunteers who received the vaccine, cases of Covid were anywhere from 50% to 95% lower than in the group of volunteers who received a placebo. On an individual basis, an efficacy rate of, say, 80%, means an immunized person’s risk of becoming ill is roughly 20% of that for an otherwise similar non-immunized person. However, a vaccine’s performance in the real world, known as its effectiveness, isn’t necessarily the same as its efficacy in a study under controlled conditions. And both efficacy and effectiveness can vary for the same vaccine across populations, time points and the regimens used; the rates are influenced by multiple factors, including the SARS-CoV-2 variants present and adherence to social and public health measures that prevent transmission of the virus.


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